Known fill level measuring devices determine the fill level according to a travel-time difference method. Travel-time difference methods utilize the physical law, according to which the travel distance equals the product of travel time and propagation velocity. In the case of fill level measurement, the traveled distance corresponds to twice the separation between the antenna and the surface of the fill substance. The wanted echo signal, thus the measurement signal reflected on the surface of the fill substance, and its travel time are determined based on the so-called echo curve, respectively the digitized envelope curve, wherein the envelope curve shows the amplitudes of the echo signals as a function of the separation, ‘antenna—surface of the fill substance’. The fill level itself results then from the difference between the known separation of the antenna from the floor of the container and the separation of the surface of the fill substance from the antenna, as determined by the measuring. Fill level measuring devices are applied in containers with heights of up to 30 m.
All known methods can be applied, which enable relatively short distances to be determined by means of reflected measuring signals. If the measurement signals are microwaves, respectively high-frequency measuring signals, then both pulse radar as well as also frequency modulation, continuous wave radar (FMCW-radar) can be used. Microwave measuring devices, which use pulse radar, are manufactured and sold by Endress+Hauser under the mark, ‘MICROPILOT’. The frequencies used by known microwaves fill-level measuring devices lie, for example, at 6 GHz, 26 GHz or 76 GHz. FIG. 1 shows schematically an apparatus for determining fill level in a container. A description of FIG. 1 is presented in the description of the figures.
An essential component of a microwave fill-level measuring device is the high frequency module. The high frequency module includes a high frequency oscillator, which produces the microwave measuring signals, and a transmitting/receiving separator, which controls the transmitting of the microwave measuring signals, respectively the receiving of the reflected microwave measuring signals.
A known method for producing high-frequency measurement signals proposes to supply a high frequency oscillator with a pulsed supply voltage. An oscillator suitable for application in fill level measuring technology has preferably a relatively low quality factor and a high output power. The low quality factor is necessary, in order within a very short time span to pass as rapidly as possible through transient behavior to reach maximum amplitude and to have a clean decay of the oscillator. The quality factor of an oscillator refers, in such case, to the rise or decay time of an oscillator as a multiple of the period of the oscillation. A relatively high output power is necessary, in order to assure reliable fill-level measurements over a range of at least 30 m.
The circuit arrangement of an oscillator is composed essentially of an active component, which produces an oscillation, and a circuit embodied for feedback, via which the desired oscillatory behavior is tuned. Known oscillators work both in the region of low as well as also in the region of high frequencies preferably according to the Colpitts, Hartley or Clapp principles. If, in the case of an oscillator with tunable frequency, a field effect transistor is used as active component, then, in the case of known circuit arrangements, the oscillation frequency and the tunable range are essentially determined by the circuitry of the gate of the field effect transistor. In such case, the variable component, which is, for example, an adjustable capacitance, is placed in the gate circuit. Usually, the adjustable capacitance is a capacitance diode, or varactor. The circuit arrangement of a Clapp oscillator is shown in FIG. 2. A description of the circuit diagram is provided in the description of the figures.